Preparation, Characterization And Application Of A Visible Light TiO₂ Photocatalyst Prepared By A Surface Chemical Modification Process

With industrialization development and population growth, the environmental contamination caused by organic pollutants is becoming an overwhelming problem all over the world. As a photocatalyst, TiO2 has the advantages of high chemical stability, high photocatalytic activity to oxidize pollutants in air and water, relative low-price and nontoxicity. However, the wide band gap of TiO2 (3.2 eV) only allows it to absorb the ultraviolet light (<387 nm) that occupies only a small fraction (3-5%) of the solar photons, which greatly limits its wide use. Therefore, we reported surface modification of TiO2 nanoparticles with several kinds of organic compounds that are able to induce shift of the absorption onset toward the visible region of the spectrum, compared to the unmodified photocatalyst.In the second chapter, the surface of titanium dioxide nanoparticles has been chemically modified with toluene 2,4-diisocyanate (TDI). The modified titanium dioxide can efficiently absorb visible irradiation. The samples were characterized by means of wide-angle X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), diffuse reflectance UV-Vis spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The as-prepared photocatalyst exhibited good photostability and photocatalytic performance in the degradation of organic compounds. For model organic pollutants (Methyl Orange, Phenol, Fluorescein), modified titanium dioxide had excellent visible-light photocatalytic performance and the surface chemistry linkage product had high photostability.Subsequently, DDAT(S-1-Dodecyl-S'-(a, a'-dimethyl-a "-acetic acid) trithiocarbonate) modified TiO2 photocatalysts were prepared by hydrothermal treatment before TiO2 crystallization. The adsorption of DDAT onto the surface of titania nanoparticles led the shifting of the onset wavelength of the optical absorption in the visible range corresponding to ligand-to-metal charge transfer transition within the surface-modified complex. The interaction of TiO2 nanoparticles with DDAT was investigated by infrared spectra. The XRD indicated that the modification process could not influence the crystallite phase of TiO2. The photocatalytic studies suggested that the DDAT modified TiO2 photocatalysts showed enhanced photocatalytic efficiency of photodegradation of 2,4-dichlorophenol compared with the as-prepared TiO2 under visible-light irradiation.Then, TiO2 was in situ surface modified in the hydrolysis of TBOT with catechol to improve its photocatalytic activity. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectra (FT-IR), thermogravimetric analysis (TGA) and UV-vis diffuse reflectance spectra (DRS) were carried out to characterize the composites with different catechol contents and different prepared temperatures. The photocatalytic degradation of Acid Orange 7 was chosen as a model reaction to evaluate the photocatalytic activities of the modified catalysts. Results show that TiO2 nanoparticles are chemical modified by catechol to lead the shifting of the onset wavelength of the optical absorption in the visible range. The modification does not change the crystalline structure of the TiO2 nanoparticles according to the X-ray diffraction patterns. Through TEM analyses it was confirmed that the catechol bonding on the surface of TiO2 were uniform distribution on the surface of catalysts. The photocatalytic studies suggested that the in situ modified TiO2 photocatalysts showed enhanced photocatalytic efficiency compared with the pure TiO2 under visible-light irradiation. The optimum preparation condition was found at a weight ratio of 4.0 wt%(Catechol/TiO2). The modified photocatalysts have excellent photochemical stability. The visible light photocatalytic activities of these samples still remain after repeatedly used for 3 times.Lastly, Titanium dioxide nanoparticles were modified by polyaniline (PANI) using'in situ'chemical oxidative polymerization method under different pHs. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy spectrum (XPS) and EDS were carried out to characterize the as-prepared photocatalysts. The photocatalytic degradation of 2,4-dichlorophenol was chosen as a model reaction to evaluate the photocatalytic activities of the modified catalysts. The modification does not alter the crystalline structure of the TiO2 nanoparticles according to the X-ray diffraction patterns. DRS spectra reveal that PANI-modified TiO2 composites show stronger absorption than neat TiO2 under the whole range of visible light. The resulting PANI-modified TiO2 composites exhibit significantly higher photocatalytic activity than that of neat TiO2 under visible light irradiation (λ> 420 nm). The optimum preparation condition was found at pH=7.0 because of PANI has the best electrical conductivity under this condition.As a new material, more and more people begin to pay attention to titania and study it. In this paper, we modified the titania through a series of organic compounds and obtained some achievement. We have improved the catalyst's ability to respond in the visible region and enhanced the efficiency of photon and its degradation rate by producing complex on its surface. But there is no end to science, to make titania widely used in environmental field, there is still need of further exploration and research.